Document use of macros inside of @mutate()

[kdpsingh]

Because macros would need to be expanded first to ensure they are properly escaped and vectorized (which isn't supported yet), we should add an example to the documentation about how to wrap macros inside of an anonymous function to ensure proper escaping and vectorization.

We discussed an example here, which we should mention in the documentation: https://discourse.julialang.org/t/trying-to-understand-macro-scope-tidier-package/103635/22?u=kdpsingh

[kdpsingh]

Thinking more about this, we can probably add functionality that will auto-expand macros using a new function like parse_macroexpand() before running parse_autovec(). This way, auto-vectorization should work normally so that anonymous functions aren't even required (though still a good idea to document them).

[kdpsingh]

Should also test if ~(x -> foo(x))(a) syntax works with anonymous functions to prevent all auto-vectorization and give users full control.